Red-colored electric lamp

ABSTRACT

The invention relates to electric lamps and more particularly to automotive signal lamps. A glass composition is described for use in electric lamps, which glass is red colored. The glass composition comprises copper between 0.1 and 2% by weight, tin between 0.1 and 2% by weight, barium between 7 and 11% by weight, and strontium between 1 and 5% by weight.

FIELD OF THE INVENTION

The present invention relates to electric lamps. In particular theinvention relates to an electric lamp having a glass envelope, the glassbeing red colored. The invention applies to incandescent lamps and moreparticularly to automotive signal lamps, e.g. indicator lamps.

BACKGROUND OF THE INVENTION

Automotive signal lamps must have specific colors defined byinternational traffic regulations. For example, automotive signallingfunctions like tail, stop, or fog lighting require red lamps. In orderto obtain the red color as defined by international traffic regulations,prior-art lamps are usually coated with a red varnish or lacquer. Adisadvantage of the prior-art lamps is that the lacquer or varnish maydegrade and may eventually peel off from the wall of the lamp envelope.Actually, these red coatings usually consist of vulnerable constituentsthat have a relative low thermal resistance. Another disadvantage of theprior-art lamps is that additional steps are necessary during themanufacturing process of the lamp for cleaning and varnishing the lampenvelope. The manufacturing cost is increased by these additional steps.

SUMMARY OF THE INVENTION

It is an object of the invention to provide an electric lamp suitablefor automotive applications, said lamp having a glass envelope, theglass being red colored.

According to the invention, these objects are achieved by an electriclamp as mentioned in the opening paragraph, the glass compositioncomprising copper between 0.1 and 2% by weight, tin between 0.1 and 2%by weight, barium between 7 and 11% by weight, and strontium between 1and 5% by weight.

No additional manufacturing step is necessary during the production ofan electric lamp according to the invention, since the specific colorpoint of the glass envelope is realized by the particular composition ofthe glass. At least, the steps of cleaning and varnishing aresuppressed. Due to its specific composition, the glass may have a redcolor point suitable for automotive signal lamps, as defined byinternational traffic regulations.

Moreover, the colored glass obtained in accordance with this glasscomposition yields a better quality glass, since the obtained red colorinherent in the glass composition stays forever, which may not be thecase of varnished glasses. Actually, the use of barium and strontiumlowers the ion conductivity of the glass, which results in athermostable color of the glass at lamp operating temperatures.

Furthermore, such a glass is particularly suitable for automotive signallamps, because the presence of barium increases the electricalresistivity of the glass, which is required for the electricalfeed-through.

In an advantageous embodiment, the glass composition comprises thefollowing constituents, expressed in percentages by weight: SiO₂ 60-72Al₂O₃ 1-5 Li₂O 0.5-1.5 Na₂O 5-9 K₂O 3-7 MgO 1-2 CaO 1-3 SrO 1-5 BaO 7-11 CuO 0.1-2   SnO₂ 0.1-2  

The SiO₂ content is limited to 60-72%. These contents, in combinationwith the other constituents, lead to a readily meltable glass. Al₂O₃improves the chemical resistance and the corrosion resistance of theglass. The alkaline metal oxides Li₂O, Na₂O, and K₂O are used as meltingagents and reduce the viscosity of the glass. MgO and CaO reduce theliquefying temperature and the melting temperature of the glass. Such aglass is lead-free, cadmium-free, and selenium-free, which elements areharmful for the environment.

The invention and additional features, which may be optionally used toimplement the invention, are apparent from and will be elucidated withreference to the drawings described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail, by way of example,with reference to the accompanying drawings, in which:

FIG. 1 is a schematic chart illustrating an example of an electric lampaccording to the invention.

FIG. 2 is a chromaticity diagram showing the characteristics in thecolor triangle of red colored glasses according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows an electric automotive signal lamp 1, also denotedindicator lamp, comprising:

a lamp envelope or bulb 2 made from red colored glass, having a wallthickness comprised between, for example, 0.3 mm and 1.1 mm,

a mount 3 comprising two lead wires 3 a and 3 b held by a bead 4 mountedinside the glass bulb 2 for supporting a coiled filament 5,

an exhaust tube 7, heated and sealed up with the mount 3 and the bulb 2,thereby forming a vacuum-tight pinch for exhausting gases from the lampenvelope 2 and for introducing inert gas into the lamp envelope duringthe manufacturing process of the lamp,

an electric contact 8 connected to, the mount 3 for the lamp to beelectrically supplied by the electric circuits of the car,

a metal cap 9 to fit in a holder of the lamp set.

In an advantageous embodiment of the invention, the starting materialsfor making the glass are quartz sand, spodumene, dolomite, thecarbonates of Li, Na, K, Sr, and Ba, CuO and SnO₂.

An electric lamp in accordance with an advantageous embodiment of theinvention has a glass bulb with a glass composition comprising theconstituents as listed in Table 1. TABLE 1 glass composition accordingto an advantageous embodiment of the invention. Composition Constituent(% by weight) SiO₂ 60-72 Al₂O₃ 1-5 Li₂O 0.5-1.5 Na₂O 5-9 K₂O 3-7 MgO 1-2CaO 1-3 SrO 1-5 BaO  7-11 CuO 0.1-2   SnO₂ 0.1-2  

In this table, the percentages by weight of the oxidic forms of theconstituents are given. This does not necessary imply that theseelements are present in the glass in this oxidic form. For example,copper in the glass might be present as metallic Cu, Cu⁺, CuO, or otherforms of copper. As a consequence, expressions like “the glasscomposition comprises element X between α and β% by weight” mean thatall the forms of element X comprised in the glass composition accountfor a percentage between α and β% by weight.

SiO₂ serves as a network former in the glass. The SiO₂ content islimited to 60-72% by weight, leading to a readily meltable glass incombination with the other constituents. If the content is below 60% byweight, the cohesion of the glass and the chemical resistance arereduced. If the content is above 72% by weight, the glass formation isimpeded and the risk of surface crystallization increases. Al₂O₃improves the chemical resistance and the corrosion resistance of theglass. Below 1% by weight the effect is too small and thecrystallization tendency of the glass increases. Above 5% by weight theviscosity and the softening temperature of the glass increase too much,which affects the workability of the glass. The alkaline metal oxidesLi₂O, Na₂O, and K₂O are used as melting agents and reduce the viscosityof the glass. They increase the electrical resistance of the glass(mixed-alkali effect). BaO has the favorable property that it increasesthe electrical resistance of the glass and reduces the softeningtemperature of the glass. Below 7% by weight, the melting temperature,the softening temperature, and the working temperature increase toomuch. Above 11% by weight, the liquidus temperature and hence thecrystallization tendency increase too much. The alkaline-earth metaloxides SrO, MgO, and CaO have the favorable property that they reducethe liquefying temperature and the melting temperature of the glass.

The glass may additionally contain some Fe₂O₃ as an impurity originatingfrom the raw materials used. Also TiO₂, ZrO₂, and MnO may be found astrace elements.

The red color of the glass according to the invention is obtained byadding between 0.1 and 2% by weight of CuO and a reducing agent, e.g.carbone or charcoal, in the glass composition. SnO₂ is also added in theglass composition.

In the presence of a reducing agent and SnO₂, copper colors silicateglasses red, which is explained by the possible precipitation of Cu inmetallic form, as will be described hereinafter.

An electric lamp in accordance with another embodiment of the inventionhas a glass bulb with a glass composition comprising the constituents aslisted in Table 2. TABLE 2 glass composition according to anotherembodiment of the invention. Composition Constituent (% by weight) SiO₂60-72 Al₂O₃ 1-5 Li₂O 0.5-1.5 Na₂O 5-9 K₂O 3-7 MgO 1-2 CaO 1-3 SrO 1-5BaO  7-11 CuO 0.6-0.9 SnO₂ 0.3-1.8 Fe₂O₃   0-0.05

With respect to the element whose weight percentage has a lower limitequal to zero, it means that the element must not be added as a rawmaterial may but yet be present in the finished glass as a result ofcontamination of the raw materials.

The glasses made with the constituents as listed in Table 1 or 2 havecharacteristics as shown in Table 3. TABLE 3 Physical properties of theglass according to the invention. Physical property Value T_(strain)(low stress relief temperature) 455° C. T_(anneal) (high stress relieftemperature) 490° C. T_(soft) (softening temperature) 675° C. T_(melt)(melting temperature) 1490° C. Specific mass 2.62 * 10³ kg · m³ Specificresistance 7.9 * 10⁶ Ω · m Linear expansion coefficient (25-300° C.)9.1 * 10⁻⁶/° C.

The glass-making process will now be described below. A batch isprepared by weighing and mixing all oxidic components mentioned above.The batch is fed to a conventional continuous melting tank. Adjustmentsof the recipe can be made by adding separate ingredients in the feedingsystem for enhancing the coloring. The melting conditions, includingtemperature and gas atmosphere, are adjusted to obtain a stableprocessing for melting, refining, and color stabilization.

In a series of complex reactions, the excess gases, including CO, areremoved from the glass melt to avoid air lines in the formed tubes.Glass tubes are made at the end of the glass manufacturing process usingthe well-known Danner process in the factory. However, the well-knownVello process could also be applied. Part of the tube is heated to makelamp bulbs.

The striking of the glass may be explained by the presence of metallicCu in crystals of colloidal size, in equilibrium with Cu⁺. Furthermore,it appears that striking requires a certain degree of nucleation, whichis why SnO₂ is added. Cu is introduced in the form of the oxide incombination with a reducing agent. In this example, the reducing agentis carbon, but any other reducing agent could be used, including Al andSi. Suitable time and temperature of striking, which depend on the basiccomposition, are best determined in a gradient furnace. For example, astriking temperature between 500 and 550° C. might be used. The time ofstriking might be between 0.5 and 3 hours.

Copper is introduced as CuO in a percentage comprised between 0.1 and 2%by weight. Larger amounts of copper are harmful, as they decreasetransmissivity in the red region. Lower concentrations do not producecoloring. Under reducing conditions, an equilibrium is obtained betweenCu²⁺, Cu⁺ and Cu. When sufficient Cu⁺ ions are present in the glass, theconcentration of Cu in equilibrium with Cu⁺ exceeds the solubilitylimits and Cu is precipitated. The red color is obtained by colloidallydispersed Cu particules in equilibrium with Cu⁺, when the bivalentcopper has disappeared. The reducing element should therefore be able toreduce all Cu²⁺. In this example, the reducing element C is chosen in aproportion C/Cu=1.67.

The function of tin may be that of a metallophilic element. As soon as anucleus of a Cu crystal has formed, it selectively attracts the Sn²⁺ions, which form an efficient barrier against further growth of themetal crystal and thus stabilizes the colloidal dispersion of the Cu andprevents further growth of the Cu atom aggregates and theiroverstriking.

FIG. 2 shows a variety of color points of red colored glass for use inthe glass envelope of signal lamps in accordance with internationaltraffic regulations, with respect to X and Y co-ordinates in thechromaticity chart. This color point may be achieved with the glasscomposition in accordance with the invention described above. It isnoted that the exact composition of the glass which gives the colorpoint depends on the production of the glass, particularly the reducingstate, and on the curing after bulb blowing.

In Europe, a red color point for automotive indicator lamps is definedby the BCE regulation, known to those skilled in the art. It correspondsto the area shown in continuous lines of FIG. 2. The red ECE-region isdefined by the following color coordinates: (0.657, 0.335); (0.665,0.335); (0.732, 0.27); (0.728, 0.27).

The SAE (Society of Automotive Engineers) has proposed another region,which is defined by the following color coordinates: (0.65, 0.33);(0.67, 0.33); (0.7367, 0.2653); (0.7164, 0.2636).This region is shown indotted lines on FIG. 2.

The GTB (Groupe de Travail de Bruxelles) commission has proposed a new,larger area which encloses both the current ECE and SAE colorboundaries. This is known as the CIE-region.

The triangles in the chart of FIG. 2 show the color points obtained witha glass of 0.5 mm thickness comprising 0.9% by weight of copper, 0.6% byweight of tin, 9.0% by weight of barium, and 3.0% by weight ofstrontium. The striking temperature is 540° C. The results are given forstriking times of 2 and 3 hours.

The drawings and their description hereinbefore illustrate rather thanlimit the invention. It will be evident that there are numerousalternatives which fall within the scope of the appended claims. In thisrespect, the following closing remark is made.

The word “comprising” does not exclude the presence of other elementsthan those listed in a claim. The word “a” or “an” preceding an elementdoes not exclude the presence of a plurality of such elements.

1. An electric lamp having a glass envelope, the glass being redcolored, the glass composition comprising copper between 0.1 and 2% byweight, tin between 0.1 and 2% by weight, barium between 7 and 11% byweight, and strontium between 1 and 5% by weight.
 2. An electric lamp asclaimed in claim 1, wherein the glass composition, expressed in percentsby weight of the oxidic forms of the constituents, comprises: SiO₂ 60-72Al₂O₃ 1-5 Li₂O 0.5-1.5 Na₂O 5-9 K₂O 3-7 MgO 1-2 CaO 1-3 SrO 1-5 BaO 7-11 CuO 0.1-2   SnO₂ 0.1-2  